Low pass filtering

[jetpack]

Hi all.
I'm thinking about changing the coupling caps in a GSG to highpass the output for a couple of frugalhorn receivers at about 80Hz, maybe a little higher. From w=1/(2pi*R*C), I get C=0.004uF to set the corner at 80Hz.
I have two questions. 1) does 0.004uF look right? 2) if I have some 0.047uF caps already, can I swap out the two following resistors (currently 475k) for 42K ones and go with the 0.047uF capacitors?
Thanks!

[Shannon Parks]

Hi all.
I'm thinking about changing the coupling caps in a GSG to highpass the output for a couple of frugalhorn receivers at about 80Hz, maybe a little higher. From w=1/(2pi*R*C), I get C=0.004uF to set the corner at 80Hz.
I have two questions. 1) does 0.004uF look right? 2) if I have some 0.047uF caps already, can I swap out the two following resistors (currently 475k) for 42K ones and go with the 0.047uF capacitors?
Thanks!

Your calculations are correct, but you don't want to lower the grid resistor as that will load down the 6SL7. Getting 4000pF is no big deal with paralleling some small value caps, just watch their voltage rating.

Shannon

[johnf]

Is there any reason to not put the cap at the input of the amp? This could make the filter easily reversible by using the cap as a plug -in.

[jetpack]

Thanks for the answer Shannon and the input JohnF. I am also curious about the best place to filter the signal.

[johnf]

If this is to do with bi-amping (subwoofer) then check out the "Good Bass" section at the top of the page here if you haven't already seen it:
http://www.nutshellhifi.com/ME2txt.html#me2
This advocates the filter at the input of the main amp (among other things - including very expensive Scanspeak driven subs)
The logic seems to be that you roll off your amp at the point where your speakers naturally roll off so you are not amplifying signal that your speakers don't deal with.
One builder I saw some years ago actually had 2 outputs from his preamp with different output caps. One for the main amp and one for the sub amp(s). The small downside to that is that you may have to change the cap if you change your main amp to one with a different input impedance.

J